The invention relates to a method and apparatus for reproducing a programmable mask on a substrate.
Standard photolithography methods reproduce a template pattern, which is pre-produced on a mask, in a fixed reproduction ratio on a substrate by means of photolithography methods. To this end, the mask is illuminated from behind, and a fixed optical unit is used to apply the image to a substrate that has a photosensitive layer on a support material. Illumination radiation selectively exposes this layer using covered areas of the mask, the mask being reproduced when the photosensitive layer is developed. Depending on the application, the reproduction can be the actual finished product, or it can also be used for selectively processing materials layers disposed thereunder corresponding to the openings produced in the layer. Such mask-reproducing photolithography methods are used for instance for producing printed circuit boards or flat screens. The masks to be reproduced include small structures, such as for instance conductors or generally geometric structures that are to be reproduced or copied onto the substrate. The typical size of such structures is a function of the application and currently is for instance 10 to 50 μm in the printed circuit board industry. Structure sizes down to 1 to 2 μm are used when producing flat screens. In current standard methods the masks are generally present in the form of films, emulsion masks, chromium masks, or the like as complete templates of the entire pattern that are transferred 1:1 to the substrate. The disadvantage of this method is that a discrete mask must be produced for each pattern that is to be reproduced, and that the masks are subject to wear over time due to loading and unloading.
Therefore, known from U.S. Pat. No. 6,291,110 B1 is a novel method in which integrated into the reproduction system and provided as the template is a programmable unit or mask into which the data to be reproduced are fed during the copying process and by which these data are depicted. The electromagnetic illumination that passes through the mask, and thus the structures to be reproduced, are reduced in size by means of a reproducing optical unit that is arranged between the mask and the substrate. A programmable mask can be smaller than the substrate, and it is nevertheless possible to expose large substrates by joining template segments programmed into the mask. Thus no large surface-area mask retention unit is necessary. The illumination unit and the reproducing optical unit may also be designed only for small surface areas. LCDs, micro-mirror arrangements or phase shift arrangements in one-dimensional or two-dimensional arrangements may be used for programmable masks, inter alia. By means of absorption, reflection, or diffraction controlled pointwise within the strip or surface area, the intensity of the radiation used for exposure can be controlled with these elements in order to produce exposed or unexposed areas. The illumination intensity can be regulated pointwise in stages or continuously, so that it is also possible to produce grayscale values.
One property of programmable masks is that they can only provide the template at a specific resolution that is determined by the size of the functional units, hereinafter called pixels. The term “resolution” is used herein to indicate the accuracy with which the edge positions of structures can be defined. While the position accuracy with which structure edges are reproduced is solely a function of the resolution limit for the optical unit when using conventional masks, the resolution of the reproduction is determined by the pixel size and the resultant object grid when employing programmable masks in combination with a 1:1 reproduction.
Another photolithography method that is described in DE 103 17 050 A 1 concerns the issue of distortion in the substrate. Generally used for substrates, in particular in printed circuit board production, are substrates that have a large surface area relative to their thickness, which is in the range of several mm (millimeters) to a few μm (micrometers). Frequently there must be a plurality of structurings on these substrates, and the production steps between these structurings can involve high temperatures differences or mechanical stresses that can lead to deformation of the substrate surface. In order to be able to expose a second layer of structures cleanly over the first layer such that the structures that belong together coincide, in this method the distortion of the substrate is calculated by means of detecting the position of alignment markings that are present using camera images and is compensated during exposure. In this method, the structures of a mask are illuminated by point and reproduced on the substrate in a distorted manner according to previously acquired data regarding the distortion correction such that the distortion of the substrate is compensated and the alignment markings for the mask coincide with those for the substrate. In this method, mask and substrate are connected to one another at a fixed distance using a mechanical apparatus and the illumination unit and the distortion-compensating reproduction unit, which transfers the image of the mask to the substrate and which is itself coupled securely to the illumination unit, move relative thereto. Movable optical components are used in the reproduction unit for compensating distortion.